Expose utils::b or add a check method to simplify jiff integration for third-party crates

[l-7-l]

Hello, I'm trying to add support for jiff to bitcode (discuss). I noticed that utils::b is private. If it were made public or if a check method were added, it would be very helpful for third-party libraries when adding support for jiff.

for example

   impl Timestamp {
        pub fn check(second, nanosecond) -> Result {
               let secs = b::UnixSeconds::check(second)?;
               let nanos = b::SignedSubsecNanosecond::check(nanosecond)?;
               if secs == b::UnixSeconds::MIN && nanos < 0 {
                 return Err(b::UnixSeconds::error().into());
               }
             }
      Ok(())
   }